T&TI last detailed plans for the reconstruction of the Connaught Tunnel over a year ago but even then it was recognised that continuing site investigation work on the state of the tunnel structure and unexploded World War II bombs possibly in the zone of influence could modify the original plans. It is likely no one envisaged how radical the modifications would have to be.
Contracts
The Connaught Tunnel is part of Crossrail contract C315, which was awarded to Vinci Construction UK along with the adjacent work on the railway route rebuild to North Woolwich on the north bank of the River Thames. This branch of Crossrail will then continue on other contracts to Woolwich and Abbey Wood through new Thames Tunnel. The route follows the route made free by the closure of the North London Line branch and national rail tracks to Custom House, closed in 2006.
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Contract C315 covers 2km of track of which half is the Connaught Tunnel, some of which is of open structure with brick arch buttresses. These lengths are being preserved, although the potentially unsafe structure of the tunnel itself cannot now be.
Vinci operates the sites under the trade name Taylor Woodrow which it uses for its civil engineering. The project manager is Paul Edwards. Project director for the client is Linda Miller; seconded to Crossrail from Bechtel.
History
The Connaught Tunnel was first completed in 1878 to remove the bottleneck of having to use a swing bridge, the base of which still exists, for rail traffic to pass through the Connaught Passage between the two dock basins. Dock traffic grew rapidly and made this idea impractical. The tunnel is a twin-track, single-bore structure until it passes under the Connaught Passage where it splits into twin tubes, each 550m long, with a vertical ventilation shaft at each end.
The increasing size of cargo ships needing to use the docks meant that the Connaught Passage into the Royal Victoria Dock needed deepening in the 1930s. In order to preserve the tunnel structure the alignment was lowered and full-circle steel rings installed, interlocking with each other to form a binocular profile. This was an early use of structural steel underground.
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By GlobalDataThe tunnel has been used for both freight and passenger traffic, the latter most recently using third-rail electrification, evidence of which has now been removed. In addition to its long-term workaday duties, the Connaught Tunnel has also appeared in a feature film – Guy Ritchie’s ‘RocknRolla’ in 2008 after rail closure.
Preparation
One of the first tunnel jobs has been the removal of old rail infrastructure, ballast and debris to exposure the brick-built invert with ribs. Once cleaned, inspected and grouted, structurally sound arches are being preserved in the approaches and single-tube sections of the tunnel.
The old Silvertown Station on the same route and contract is being demolished.
London’s Docklands was a main target of bombing in World War II. Although maps have been compiled of known locations, due mainly to the spotting activities of ARP wardens during the Blitz, there is still a strong possibility that any underground construction activity might uncover or disturb one, the consequences of which could be disastrous. Although it is not often mentioned, previous tunnelling works in east London’s Docklands have hit them before. Therefore specialist contractor Fugro undertook a survey of the ground adjacent to the rail route lest any bombs might be disturbed unwittingly. Freely available mapping data, and the use of sophisticated instrumentation such as magnetometry and ground-probing radar have made the task a bit easier, but not without risk.
Despite a direct hit on the tunnel, necessitating repairs to the brick lining, the tunnel crown structure held together, even though other bombs fell in the vicinity.
Sonar shock
One of the survey measures taken during site investigation was a detailed sonar survey of the passage of the existing Connaught Tunnel under Connaught Passage between the Royal Victoria and Royal Albert Docks. It was know historically that the bottom of the passage would have been subject to possible interference from larger ships, despite deepening and tunnel restructuring work in the 1930s, but the extent of any damage had never been properly quantified.
Using a relatively new multi-beam sonar method carried on the special vessel ‘Galloper’, the Port of London Authority, working for subcontractor Abwood, produced a detailed plot of the Connaught Passage. The system employs the equivalent of 512 probes, including side and back echo measurement, and producing a multitude of measurements. Compared with the conventional survey results of the tunnel interior, this showed that the tunnel cover was much less than expected including a potential area of breach. According to project manager Paul Edwards, at the worst point the cover was only 300mm, and that was broken concrete. So the risk of inundation was substantial even before reconstruction.
Since it is necessary to enlarge the passage for the new Crossrail rolling stock it was decided that the risks involved in underground work were too great, if not impossible.
Abwood is providing a number of other support services to the project as subcontractor including marine divers, surveyors and silt removal. Among the debris found in the docks are some of the legendary ‘over 100’ cars, although all have already been inspected by police.
Water, water
"The high water-table can be a nightmare," says Linda Miller. Despite its upper parts resting in clay, the original tunnel has a long history of leaks, whether from the docks or the underlying mark. The groundwater pressure on the brickwork has been controlled by a permanent drainage and pumping system, currently still in use.
The natural geomorphology of the area is a swamp, in close proximity to the River Thames, so all drainage has to be artificial. There are also tidal effects on groundwater pressures. Whilst a bed of clay is present at the tunnel horizon, it is overlain by an Upper Aquifer of river terrace gravels, and underlain by marl.
A large proportion of the project is concerned with water control in various forms, especially as the cofferdam excavation has to be kept dry for the new temporary works.
Specialist contractor WJ Groundwater has been installed dewatering wells around the Connaught Passage to lower the watertable, and carrying out pumping tests to determine groundwater flows. Lowering of the watertable will also allow the deepening of the existing pumping shaft.
As well as minimising water nuisance and making construction practical, it will also reduce groundwater pressures. This is particularly relevant in connection with old structures such as the existing tunnel and dock-walls, the condition of which has not yet been fully determined. The construction designers have requested continuous monitoring of water levels and pressures.
The new permanent drainage arrangements will include a new connection, probably microtunnelled, from the new tunnel sump to the pump-house shaft, which will have been deepened by a further seven metres to 25m.
Four larger units will replace the existing pump range of three units. The space required for installation and future maintenance means that the existing Victorian pump head-house is too small. Although final negotiations are still under way it is intended that it will be dismantled brick by brick and donated to the Newham local council for re-erection adjacent to the intended mooring of a preserved steamship, the SS Robin, perhaps as a ticket office.
Strengthening
In view of uncertainties about ground behaviour when it is subjected to changes in water levels and pressures, it is being ‘stiffened’, by the Soletanche Freysinnet group, with a series of 3,000 controlled modulus columns (CMC) so that the behaviour can be better predicted. The columns are excavation by specially designed auger-borers and are pumped with PFA grout.
A similar operation had been undertaken in equivalent conditions in the area for the Channel Tunnel Rail Link excavation (HS1).
Reflecting the original method of construction, the new Connaught Tunnel will be built by cut and cover methods, necessitating the use of cofferdam through the Connaught Passage. Such reconstruction will also allow the tunnel to be deepened and enlarged more easily.
Cofferdam
Agreement was reached to avoid disrupting the 2013 London Boat Show, which is held with many visiting boats and ships in the Royal Dock. Consequently installing of the cofferdams in the Connaught Passage will not commence until February 2013. Although this will allow plenty of time for ground reinforcement work and work on the tunnel approaches, it also means that all works have to be completed within a six-month period in order to stay of schedule.
The cofferdam across the narrow waterway will be formed with two rows of interlocking sheet piles filled back with grouted sand and gravel. They will stretch approximately parallel with the pedestrian bridge over the passage and to the east of the Connaught Bridge carrying the main A1020 highway. This presents quite a logistical challenge to manage the site, with restricted access and space available, both horizontally and vertically.
Once completed, the temporary cofferdam will allow the water in between to be pumped out and excavation of the remaining ground and removal of the tunnel steel rings to take place. Excavators can be mounted on the dock wall and cofferdam themselves. As the ground will have been strengthened, pressure on the dock walls will be minimised, but the excavation will also be supported with temporary struts.
Archaeology
Another non-construction factor that has to be considered is the ancient history of the area. Provision for allowing archaeological ‘digs’ before covering with new construction is built into all relevant Crossrail contracts. The rail route being refurbished actually follows an old Roman road, which in its turn is also believed to have followed a prehistoric pathway.
Consequently there are digs going on at selected sites on the route. A recent press visit was unable to visit the excavations for safety reasons as they had uncovered an unmapped 10kV live electrical power cable. The high water table also interferes with archaeological excavations as well as the main tunnel work. Nevertheless, evidence of settlements dating back 6,000 years has been found.
Community and environment
All work is subject to planning and environmental restrictions, especially in view of the residential, commercial and transport link nature of surrounding properties. Excluding setup and shutdown periods, normal working is from 8:00 to 16:00 on weekdays and 08:00 to 13:00 on Saturdays. All compound areas are surrounding by high hoardings that are also said to have the effect of reducing dust emission. Both this and noise controls are in force.
There is liaison with the London Borough of Newham local authority on environmental as well as official planning matters. The change to cofferdam construction has resulted in the need to apply to extend the extent of the working area slightly around the Connaught Passage to allow for the extra surface equipment required.
There also needs to be liaison with a wide variety of other stakeholders, especially in the London Olympics year and with the wide variety of other construction work going on in London’s Docklands including, it was jokingly said, the fish in the docks.
Last year the Vinci Construction UK team at the Connaught Tunnel won a silver award in the UK’s independent Considerate Constructors Scheme. This is an industry run initiative that assesses and monitors sites against a code of practice, and commits them to being clean, respectful, safe, environmentally conscious, responsible, accountable and, overall, a good neighbour.
Progress
The contract sites were mobilised from April 2011 with the main work commencing at the end of September last year. Work on the surface route, both civils and railway work, has been progressing well since commencement. Following extensive site investigation, the tunnel itself has now been cleared, and water control measures being installed as above. Despite the changes in the approach to reconstruction, the contract is still understood to be on schedule, although on the critical path, with the tunnel due for completion by October next year. Connaught Tunnel scored 36.5 out of 40 points compared to a UK average of 31.
